The linearization of class A/B high power amplifiers has been a challenge faced by the radar and telecommunications industries for many years. Current linearization schemes include use of a predistortion circuit, a feedback error compensation loop or a feedforward error compensation loop.
With a predistortion circuit, the input to the amplifier is pre-distorted based on the characteristics of the amplifier to compensate for the distortion introduced by the amplifier. However, this technique offers very limited improvement as it ignores memory effects and is generally considered by the power amplifier community to be primarily an addition to a feedforward error compensation loop.
With a feedback error compensation loop, the input and output signals of the amplifier are compared and used to adjust the input to the amplifier. Many variations of feedback error compensation loops exist. For example, the adjustments can be proportional to magnitude and phase error between the two signals, or can be proportional to in-phase and quadrature amplitude error.
One problem with feedback error compensation loops is that the level of linearization achieved is somewhat modest due to limitations imposed by stability criteria. This is particularly true when the feedback error compensation loop is applied to a power amplifier operating in the high frequency range.
With a feedforward error compensation loop, the output of the amplifier is adjusted. Generally, a feedforward error compensation loop generates an error signal by comparing the input signal to the amplifier and the output signal produced by the amplifier, and then amplifying and filtering the result to obtain an error correction signal. By vectorial summation of the error correction signal and the output signal produced by the amplifier, the error or distortion introduced by the amplifier can be reduced. Specifically, the distortion is reduced because the distortion components in the error correction signal are in antiphase with the distortion components in the output signal produced by the amplifier.
While a higher level of linearization can be achieved with a feedforward error compensation loop, the feedforward error compensation loop is highly sensitive to device aging or component drift and the amplitude and phase matching must be maintained to a very high degree of accuracy over the band of interest.